The invention has been developed from a process for the control of blood pumps in an extra-corporeal circuit of a single needle arrangement as is known from the Journal Medizintechnik, Vol. 106, S/86, pages 146 to 148 and is also directed to an apparatus therefor. It is particularly suitable for an extra-corporeal dialysis circuit.
The use of the Single-Needle-Arrangement or other arrangements with a single needle in dialysis has been known for a substantial amount of time. In previous developments, attempts were made to improve the efficiency of the general procedure and to solve the various different problems which arose, such as for example, the improvement of the control of the pumps for carrying out the work cycle. There is basically a common problem with such single needle arrangements which occurs with all known arrangements and which has previously not been addressed because other difficulties and disadvantages appeared to be more important. This problem relates to the recirculation during the procedure of single needle dialysis.
Previously known arrangements utilize either one or two pumps and additionally, one or more clamps, wherein procedures are known wherein the dialyzer is provided with between two pumps or in which it is provided downstream from the venous pump. There are also arrangements in which there are provided additional chambers partially filled with air which can take the form of a surge tank, or expansion chamber, or bags, which can be provided before the arterial pump, between the pumps, or after the venous pump. These known arrangements are activated by controls wherein controls are known whose activation is are time-time dependent, pressure-pressure dependent, pressure-time dependent, volume-volume dependent, or pressure-volume dependent.
In several of the known arrangements, there is utilized a single canula or a monolumenar catheter which is connected to the patient. From this canula or catheter, arterial and venous portions of the tube system are branched off, wherein these branches can be uncoupled from the rest of the system, either by an occluding pump or by a clamp. Furthermore, it is common to all arrangements that a more or less greater part of the already dialyzed blood is again pumped into the extra-corporeal circuit before the dialyzed blood returns to the body. This procedure is known as recirculation. It is discussed in considerable detail in the Journal "Medical and Biological Engineering and Computing", Volume 17, September, 1989, pages 578 to 582.
The recirculation of the blood can have several causes. Reference will be made to the conditions in the fistula. Different streaming conditions will occur depending upon whether the determinative flow resistance to the blood stream is upstream, that is to say, on the arterial side or downstream, that is to say, on the venous side of the fistula. If the resistance is arterial, then during the arterial pumping phase, more blood is removed from the fistula than is delivered arterially. This has the result that a portion of the blood which is pumped into the area upstream of the canula during the venous phase, is sucked back during the arterial phase and is thus recirculated.
With respect to the flow conditions in the canula, it must be noted that at the end of the venous phase, the canula is totally filled with cleaned blood. In the next arterial phase, this is again sucked into the extra-corporeal circuit and thus recirculated. Attempts were made to diminish this problem by providing the canula and the catheter with smaller fill volumes. However, this did not serve to solve the problem of recirculation in a satisfactory manner.
A further aspect of recirculation is due to the hose system utilized. During the venous pump phase, the entire hose system which lies between the venous pump and the arterial pump or clamp is subjected to the venous bath pressure. Because of the flexibility of the hoses and the air buffer present in the arterial pressure probe, the available volume of this part of the extra-corporeal tube system increases and the hose system balloons out. At the end of the venous phase an additional volume of cleaned blood is stored in the tube system between the venous clamp and the arterial pump or clamp, opposite that in a pressure-less condition. In the immediately following arterial phase, this volume is recirculated. Since during this arterial phase an under-pressure develops, the available volume is reduced with respect to the pressureless condition. Thus, an additional amount of cleaned blood is pulled out of the venous hose system and thus recirculated. This condition is repeated in each cycle. Suggestions have been made to solve this problem by utilization of additional clamps or specially constructed hose lines. For practical reasons, none of these suggestions have been put into practice since the additional clamps must be provided directly on the patient which considerably interferes with his mobility. The use of special, that is to say, non-expandable hose materials, has considerable disadvantages including not only economic but technical ones.
A further factor which determines the recirculation results from electronic or mechanical delays in the arterial and venous control circuits. Thus, it can occur that the arterial pump is already activated while the venous pump is still in operation or the venous clamp is still open. In the previously known arrangements, no ways have been found to avoid these effects by which the recirculation could, in practical ways, be reduced or prevented.
Recirculation has a decisive influence on the effectiveness of the extra-corporeal circuit and thus upon, for example, the dialysis, as the volume of fluid running through the dialyzer is greater than the actual blood amount which leaves the fistula, by the recirculation portion.
Thus it would be desirable to provide a process and an arrangement of the aforementioned type which substantially avoids the negative effect of recirculation on the effectiveness of the system and thus improves its efficiency and in which the arrangement due to its simpler construction and safer mode of operation is so operated that an amount of blood timewise, independent of the recirculation, can be abstracted from the patient and returned to him.